To Ottobald, I had the same problem as you, tried and tried, so what i did was shortend my jtag cable, made sure all wires were separated from each other and then use the following msdos string

wrt54g -flash:wholeflash /skipdetect /fc:1 /instrlen:5 /alienable

you can see that fc:1 was placed after skipdetect and not at the end as you have, as if i am not wrong msdos has its problems, and the way i think the program works is, that first it gets the flash file then it skips detection gets the flash type then does the installation procedure and last enables the ali chip.

Iam not a programmer so i could be wrong but that is how it worked for me on a windows XP amd 1.8

hope you get yours going